1. Field
Embodiments of the invention relate to a servo channel for tape drive systems.
2. Description of the Related Art
In timing-based servo (TBS) systems, recorded servo patterns consist of magnetic transitions with two different azimuthal slopes. Head position is derived from the relative timing of pulses, or dibits, generated by a narrow head reading the servo patterns. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). A specification for the servo format in current tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified, can be found on the World Wide Web (www) at ultrium.com. Traditionally, the detection of LPOS information bits is based on the observation of the arrival times of the shifted dibit peaks within the servo bursts at the servo reader output (R. C. Barrett, E. H. Klaassen, T. R. Albrecht, G. A. Jaquette, and J. H. Eaton, “Timing-based track-following servo for linear tape systems”, IEEE Transactions on Magnetics, Vol. 34, Issue 4, Part 1, pp. 1872-1877, July 1998). In an alternative solution, optimum detection of LPOS bits is performed by a matched-filter detector (G. Cherubini, E. Eleftheriou, R. Hutchins, and J. Jelitto, “Synchronous Servo Channel for Tape Drive Systems,” Filed as IBM Docket TUC920060028US1, Jul. 30, 2006).
In certain prior-art architectures, however, estimates of the lateral servo reader position (y-position) and tape velocity are directly obtained by monitoring the peak-arrival times or the zero-crossing instants of the dibits of the servo bursts. The filtering for the servo reader signal used for the computation of the estimates is typically achieved by an anti-aliasing low-pass filter (LPF) in the analog domain, prior to analog-to-digital conversion. Unfortunately, at low tape velocities a fixed low-pass filter exhibits significant excess bandwidth, which leads to a large noise level, and may greatly reduce the reliability of the computed estimates, depending on the tape velocity.
The minimum (Nyquist) bandwidth of the servo reader signal is about νx/s, where “νx” denotes the tape velocity and “s” denotes the minimum distance between magnetic transitions of the recorded servo patterns. To mitigate the excess noise problem, in current tape drives the clock frequency of the analog-to-digital converter (ADC) sampling the servo reader signal is selected from a finite set of frequencies, which are generated by a phase-locked loop. This approach has the requirement that the bandwidth of the anti-aliasing filter be selectable, which leads to increased complexity. A further disadvantage arises if the bandwidth of the servo reader signal for the minimum cruise velocity of the tape is significantly smaller than the minimum available clock frequency of the ADC. In this case, which is found in practice, excess noise bandwidth cannot be eliminated at low tape velocities. Another drawback of this approach is that a variable clock rate would be required during tape acceleration and deceleration to guarantee minimum-bandwidth filtering.
A possible solution to the excess noise problem is represented by the inclusion of a filter, either in the analog or in the digital domain, with a variable bandwidth proportional to the tape velocity. Thus, the noise spectral components above the Nyquist frequency are eliminated without aliasing of the servo reader signal, which carries the relevant information for the track-following and reel-to-reel servo systems of the tape drive. Such a solution, however, leads to a significant increase in the complexity of the implementation of the servo channel. In fact, the variable-bandwidth filter should not only accommodate the various signal bandwidths associated with the cruise velocities of the tape, but should also be able to continuously vary the servo-reader signal bandwidth during tape acceleration and deceleration. FIG. 1 illustrates an architecture of a servo channel 100 according to the prior art, including a digital filter with variable bandwidth 112 to eliminate out-of-band noise spectral components. In particular, in FIG. 1, an anti-aliasing filter 102 receives input from a servo reader (not shown). The output of the anti-aliasing filter 102 is routed to an ADC 110 in the servo channel 100.
The output of the ADC goes into a variable-bandwidth low-pass filter 112. The variable-bandwidth LPF 112 varies bandwidth based on velocity to obtain optimal filtering. The output of the variable-bandwidth LPF 112 is routed to a servo channel signal interpolator 114 (also referred to as an “interpolator”), a monitoring and control component 116, a peak-arrival time component 118, and a zero-crossing time component 120. The output of the interpolator 114 is routed to the matched-filter LPOS detector 122 and the monitoring and control component 116. The output of the monitoring and control component 116 is routed to a time-base generator 124, to the matched-filter LPOS detector 122, to the peak-arrival time component 118, and to the zero-crossing time component 120.
Notwithstanding conventional solutions, there is a need in the art for velocity-independent optimum filtering of servo-reader signals in tape drive systems by a servo channel, which allows reliable recovery of longitudinal position information as well as estimation of tape velocity and head lateral position even during tape acceleration and deceleration.